The invention relates to an R.F. system for providing an at least approximate indication of the direction in which radiation is incident on the system from a distant source over an operating range of directions of incidence relative to the system. The R.F. system may be associated with means which tend to align the R.F. system with the direction of incidence of the radiation. The R.F. system may have a broad instantaneous bandwidth, for example an octave or more, and may furthermore be adapted to respond only to radiation having one or more characteristics satifying certain criteria, such as a frequency substantially equal to a predetermined value.
A known R.F. direction-finding system is that used in amplitude-comparison monopulse radar. This typically comprises a pair of parallel feeds arranged symmetrically in a reflector, giving respective narrow main-lobe radiation patterns with mutually-inclined respective axes crossing in front of the reflector. The outputs of the feeds are connected to two input ports of a hybrid ring; signals representing the algebraic sum of and the algebraic difference between the signals at the input ports appear respectively at two output ports of the hybrid ring. After translation to an intermediate frequency (I.F.), the signals derived from the output ports are subjected to further processing, including detection in such a manner as to provide a quantity whose magnitude is substantially independent of the amplitude of the incident radiation and is representative of the angle between the direction of incidence and boresight. Directions which are on opposite sides of boresight are distinguished by reference to the phase of the "difference" signal relative to the "sum" signal, there being a change of 180.degree. in this relative phase on passing through boresight (the axis of symmetry of the reflector).
In monopulse radar, a broad instantaneous bandwidth is not required for the detection system, since the frequency of the transmitted signal is known at least approximately. However, the inherent limitation to a fairly narrow bandwidth due to the down-conversion to an I.F. means that such a system is unsuitable if it is desired to have a broad instantaneous bandwidth. Furthermore, for directions of incidence close to boresight, there may be difficulty in determining on which side of boresight the direction lies, since the difference signal tends to zero at boresight.
Another direction-finding system, which is known from UK Patent GB No. 2 037 116 B and which may have a broad instantaneous bandwidth, comprises an array of more than four antenna elements coupled to respective input ports of a microwave power dividing and phase transforming matrix. Phase comparators (which must operate at R.F. if the system is to be broad-band) are coupled to output ports of the matrix. An unambiguous representation of bearing angle can be derived from the phase comparators. The required matrix is highly complex and very expensive, and will generally necessitate a respective R.F. amplifier between each antenna element and its respective matrix input port if the system is to have adequate R.F. sensitivity for signal levels likely to be encountered in practice. If the bearing representation is to be accurate, these amplifiers must track accurately in phase and at least fairly accurately in amplitude, further increasing the cost and complexity of the system.
A further known direction-finding system suitable for use over a broad bandwidth comprises an array of identical broadband antennas with their respective boresights disposed at regular angular intervals about a common point, and means for comparing the relative magnitude of signals derived from each pair of adjacent antennas. Even if the antenna radiation pattern is carefully selected, such a system tends not to be very accurate.